In general, in electrophotography, the surface of a photoconductor drum is charged by a method such as corona discharge, followed by exposure using a laser etc. to form an electrostatic latent image. The formed electrostatic latent image is developed with a toner so as to form a toner image. The formed toner image is transferred onto a recording medium to obtain an image with high quality. The toner used for formation of a toner image is typically toner particles (toner base particles) with an average particle diameter of 5 μm or larger and 10 μm or smaller produced by mixing a binder resin such as thermoplastic resin with components such as a colorant, a charge control agent and a release agent, followed by a kneading step, a pulverization step, and a classification step. For the purpose of providing flowability or suitable charging performance for the toner, and/or for facilitating cleaning of the toner from the surface of the photoconductor drum, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner.
A two-component development method using a toner and a carrier such as iron powder, and a magnetic single-component development method using a toner containing magnetic powder inside the toner without using a carrier are known as dry development methods to be employed in various forms of electrophotography which are used in practice. Toners containing magnetic powder used in the magnetic single-component development method (hereinafter, also referred to as magnetic toner) have merits including low cost and excellent durability.
Furthermore, toner is required to have a smaller particle diameter due to the recent demand for higher image quality. By allowing the toner to have a smaller diameter, reproduction of thin lines is improved, and thus the image quality of the formed image is improved.
However, in the toners whose particle diameters are smaller, charge control agents and release agents are often contained in the toners in a state in which they are separated from the toner particles. Therefore, by using the toners whose particle diameters are reduced, a filming phenomenon occurs in which toner components are attached onto the surface of a photoconductor drum. When the filming phenomenon occurs, images having a desired image density accordingly do not tend to be formed easily, and image defects such as fogging tend to appear in the formed images in some cases.
As regarding the magnetic toner, as a toner in which problems caused by such a filming phenomenon have been resolved, a magnetic toner, including at least a binder resin, a magnetic powder and a charge control agent, in which an elution amount C (g/g) of the charge control agent measured by a certain method and a specific surface area Sw (cm2/cm3) obtained from the weight average diameter satisfy a predetermined relation, has been proposed.
However, in the above-mentioned magnetic toner, a selective development in which a toner having a smaller particle diameter is preferentially developed tends to occur. When image formation is carried out repeatedly for a long time, since toner particles with smaller diameters are consumed preferentially because of the selective development, the average particle diameter of toners in a developing device becomes larger. Consequently, by using the above-mentioned magnetic toner, image quality of thin lines formed after repetitive image formation easily deteriorates as compared to images of thin lines formed in the early stage.